An updated DHT preamp
It’s been a while since I played with preamps. Here is my third iteration of the 26 preamp (please refer to previous version Gen 1 and Gen 2). I love the sound of this valve as many of you out there. I found my previous build a bit inflexible to modify bias points and play around so decided to go back to the workshop and update my preamp, again. The design is very simple as you can see in the figure below. I’m reusing an existing supply but you can get away with a 200V HT and probably a 15V raw DC for the filament circuit. Key characteristics are:
- Filament bias as I’m a fan of it, no output or input caps sir!
- Added a Salas SSHV2 regulator to provide HT voltage flexibility and vary the operating point of the 26. The SSHV2 is extremely quiet so will give a clean HT supply to the 26. Yes, the lovely colour of the glow valves are lost in this version 🙁
- The Kelvin capacitor C1 (ala Morgan Jones) will help keeping Salas noise input low, specially HF.
- R1 is a simple resistor to measure the anode current. It is located in a handy place to allow easy measurements
- T1 is the Lundahl LL2745 in Alt R mode for 5.6:1 ratio. This provides the lowest gain and the lowest output impedance. In my new system, this preamp is more a line stage as don’t need further gain.
- SW1 is helpful in case ground loops are to be broken
- The Rod Coleman regulators are set to 800mA to starve the DHT and reduce microphonics and distortion. I found 800mA to be better sounding than my previous 760mA.
- P1 is my stepped attenuator and R2 will help providing grid bias when breaking before making action of the attenuator.
- RF is 2 pairs of 10Ω/20W parallel/series wirewound.
The preamp is extremely quiet. The below measurement is with a noisy Sylvania ST valve which picks up significant 50Hz hum. Notice that 100Hz harmonic is very very low (can’t be seen at -100dB). Distortion at 5Vpp input (8Vpp output) is lower than 0.03% which is what you would expect from a 26. With a good selection of valves you can get this down to 0.02% for sure:
The most important part of the build work is grounding and avoiding ground loops. A combination of star grounding is recommended. I do the following star ground combination:
- Input stage
- Filament bias return
- Output stage
You definitely want to avoid the filament current ground introducing hum in the output stage so you want to keep the three star ground points separate.
Listening to the 26 is a fantastic experience. If you haven’t built a DHT preamplifier then do it. The detail and colour of this stage is unique. I found ST valves being more detailed on the top end, a richer treble compared to the Globe ones. Globe ones are sweeter and mellow. Definitely prefer globes for classical and ST for rock and Jazz.
The preamp is extremely quiet. Best build so far, I can’t hear hum on my high-efficiency FE167E full-range speakers and that is a real challenge.
Frequency Response (updated Feb 2014)
Here is the frequency response of the preamp. There is no Zobel compensation so you can see the HF response to present a dip at very high frequency. Either way, it’s flat overall with a lovely response from 10Hz to 30KHz (-3dB points)